Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Towards the upgrade of the ASTRI structure following the consolidation of the requirements Enrico Giro – INAF OAPD for the ASTRI Collaboration & the CTA Consortium 9° ASTRI general meeting

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Outline A bit of history Requirements Pointing concepts for Cherenkov telescopes Next steps Redesign and pointing performances

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb A bit of History ASTRI has been funded since 2011 for three years. All contracts had to be signed before the end of Contracts for design phase started in 2011, document delivery was in early Building phase started at the end The telescope of Serra La Nave has been built on requirements defined by CTA in (when defined). For this reason changes in requirements later than 2012 were not taken into account on prototype.

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Pointing performances can be obtained online or post calibration. Astronomical telescopes typically need an online calibration because of the small Field of View (FoV) of their focal instruments and use of slits for spectroscopy. In Cherenkov data acquisition the pointing performances are typically post calibration. Post calibration needs to track the real position of the telescope during event triggering. Pointing performances

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Requirements Error budget on pointing evolved since But just now we are still “consolidating” requirements. This was a main issue in the design of ASTRI prototype. It is very difficult to deal with requirements and if evolving and if overestimated. At the present these are the requirements. Working conditions Ambient temp. range-15deg +25deg Gradient7.5 deg/h Max wind36 km/h El. range deg Favorable conditions Gradient3 deg/h Max wind20 km/h El. range35-85 deg Pointing CTA< 5 arcsec per axis (systematic) Pointing SSTs Pointing 1 SST< 7 arcsec (rms space angle) Are these ones compatible?

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Requirements (II) Just a comparison: An optical telescope “old fashion” as the ESO 3.6 m has a pointing error of 5 arcsec (online calibration). The telescope is INSIDE a dome. Wind limit OUTSIDE is 72 km/h. Estimation on the secondary could be about 25% of outside wind (18 km/h) and 7% on primary cell (5 km/h). The comparison is not fully unfair because ASTRI is a 2 mirror telescope.

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Requirements (III) ( Is it this the origin of pointing requirement?

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb MAGIC calibration system A CCD mounted on the center of the dish permits to image the camera focal plane, its LEDs, the sky and except during observations, the target in the center of the focal plane. Differential distances between the target on the focal plane, the target in the sky and LEDs permits to evaluate the real pointing of the telescope. Data acquisition of the CCD camera must sampled with the same frequency we want to know the pointing performances (i.e. at least at the main oscillation frequency).

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb SST 2M telescope and pointing The new configuration of ASTRI SST-2M implies to reconsider the strategy to calibrate the pointing of the telescope. A direct imaging of camera and sky is difficult and moreover it cannot track for tilts of secondary mirror impacting on pointing performances. Three options are possible: Using the Cherenkov camera to have a snapshot of the stars in the field of view during data acquisition (variance method) Using a devoted CCD camera in the central PDM of the focal plane as “guiding” camera for pointing purposes An online calibration in the same way used for astronomical telescopes.

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Direct imaging with Cherenkov Camera Pros No additional hardware to be installed The full focal plane is used for scientific acquisition High sensitivity in detector response Cons Low focal plane sampling with respect to pointing requirement Need of accurate metrology for the camera focal plane High noise due to unresolved background stars

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Credits: O. Catalano

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Direct imaging with Cherenkov Camera (III) DSS field of the CRAB convolved for ASTRI PSF

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb CCD camera in the central PDM Pros Devoted hardware for pointing High sensitivity in detector response High focal plane sampling with respect to pointing requirement Cons High Noise due to unresolved background stars Limited field of view for sky coverage Central FoV subtracted to scientific acquisition

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Online Calibration In this case the same strategy of the optical telescopes is adopted. Calibration of flexures (using for example Tpoint) on a stiffer telescope permits to know in real time where the telescope is pointing without imaging the FoV. Pros The telescope position is know in real time Reliable system In the case of SST-2M a safer way to achieve requirements Cons Weight of the telescope Structure design pointing oriented and not PSF oriented

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Next steps In case of negligible noise, expected centroid performances are of the order of 6 arcsec. But this is not a fully realistic case. So it is will be necessary: To use synthetic images originated from GSC2 to have a realistic simulation of the ASTRI FOV with at least a limit magnitude of 14 th. For the case of variance method the real geometry of the Cherenkov camera must be know. The real shape of the PSF must be know (from prototype characterization) also in windy conditions. To simulate small shifts (~ 6 arcsec) in synthetic images to test the sensitivity of the astrometric solution To estimate sky coverage for typical targets necessary to reach pointing requirements. Check estimations directly on the prototype.

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Redesign and pointing performances ASTRI structure (GEC) has been designed for online calibration. In fact at design phase we had not: estimation of sensitivity for pointing calibration with the variance method Mechanical space for a CCD camera in the center of the FoV For this reason masses of prototype are on the side of the astronomical telescopes Miniarray redesign have to deal with new requirements (Empty column)

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb On September 2014 a talk of Punch was on “CTA Questions arising on the Wind Requirements impact on Observing time”. Project Office is thinking to change requirements? Is it really necessary? Is it correct relating the observing time for favorable condition? Could be a better approach to calculate how many sources need for favorable conditions? PLEASE In few months we are going to redesign the telescopes for the miniarray, so we NEED at redesign kick-off: To be sure pointing requirements are consolidated To be sure that requirements are not overestimated In a different situation we have only one chance: We have to select requirements we consider correct Redesign and pointing performances (II)

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb Summary Pointing performances are a critical issue for mass dimensioning of the SST-2M. Different pointing strategies could influence dimensioning of the main structure of the telescope The present design of the prototype has been driven by old requirements defined well before the building phase. An accurate and balanced definition of requirements on pointing is an essential aspect of the redesign phase aiming to decrease the mass of the SST-2M. In few months the redesign phase will start and at that point the final requirements must be settled.

Enrico Giro – INAF – OAPD - ASTRI general meeting #9– Bologna, 24 Feb N. Surname – Affiliation – Conference – Place, Date 19 THANKS!